Radiolucent spinal fusion cage

ABSTRACT

An improved ceramic bone graft is provided for human implantation, particularly such as a spinal fusion cage for implantation into the inter-vertebral space between two adjacent vertebrae. The improved spinal fusion cage includes a substrate block of high strength ceramic having a selected size and shape to fit the anatomical space, and a controlled porosity analogous to natural bone. The substrate block is coated with a bio-active surface coating material such as hydroxyapatite or a calcium phosphate to promote bone ingrowth and enhanced bone fusion. Upon implantation, the fusion cage provides a spacer element having a desired combination of mechanical strength together with osteoconductivity and osteoinductivity to promote bone ingrowth and fusion, as well as radiolucency for facilitated post-operative monitoring. The fusion cage may additionally carry one or more natural or synthetic therapeutic agents for further promoting bone ingrowth and fusion.

This application claims the benefit of U.S. Provisional Application No.60/287,824, filed May 1, 2001.

BACKGROUND OF THE INVENTION

This invention relates generally to improvements in bone grafts such asspinal fusion cages of the type designed for human implantation betweenadjacent spinal vertebrae, to maintain the vertebrae in substantiallyfixed spaced relation while promoting interbody bone ingrowth and fusiontherebetween. More particularly, this invention relates to animplantable bone graft such as a spinal fusion cage having an improvedcombination of enhanced mechanical strength together with osteoinductiveand osteoconductive properties, in a ceramic-based device thatadditionally and beneficially provides radiolucency for facilitatedpost-operative monitoring.

Implantable interbody bone grafts such as spinal fusion devices areknown in the art and are routinely used by spine surgeons to keepadjacent vertebrae in a desired spaced-apart relation while interbodybone ingrowth and fusion takes place. Such spinal fusion devices arealso used to provide weight bearing support between adjacent vertebralbodies and thus correct clinical problems. Such spinal fusion devicesare indicated for medical treatment of degenerative disc disease,discogenic low back pain and spondylolisthesis. These conditions havebeen treated by using constructs, typically made from metals such astitanium or cobalt chrome alloys such as used in orthopedic implants,and allograft (donor) or autograft (patient) bone to promote boneingrowth and fusion.

Typical interbody spinal fusion devices, such as plugs for example, havehollow or open spaces that are usually filled with bone graft material,either autogenous bone material provided by the patient or allogenousbone material provided by a third party donor. These devices also havelateral slots or openings which are primarily used to promote ingrowthof blood supply and grow active and live bone. These implants may alsohave a patterned exterior surface such as a ribbed or serrated surfaceor a screw thread to achieve enhanced mechanical interlock betweenadjacent vertebrae, with minimal risk of implant dislodgement from thesite. See, for example, U.S. Pat. Nos. 5,785,710; and 5,702,453. Typicalmaterials of construction for such interbody spinal fusion devicesinclude bio-compatible carbon fiber reinforced polymers, cobalt chromealloys, and stainless steels or titanium alloys. See, for example, U.S.Pat. No. 5,425,772.

Most state-of-the-art spinal fusion implants are made from titaniumalloy and allograft (donor) bone, and have enjoyed clinical success aswell as rapid and widespread use due to improved patient outcomes.However, titanium-based implant devices exhibit poor radiolucencycharacteristics, presenting difficulties in post-operative monitoringand evaluation of the fusion process due to the radio-shadow produced bythe non-lucent metal. There is also clinical evidence of bone subsidenceand collapse which is believed to be attributable to mechanicalincompatibility between natural bone and the metal implant material.Moreover, titanium-based implant devices are primarily load bearing butare not osteoconductive, i.e., not conducive to direct and strongmechanical attachment to patient bone tissue, leading to potential graftnecrosis, poor fusion and stability. By contrast, allograft boneimplants exhibit good osteoconductive properties, but can subside overtime as they assimilate into natural bone. Further, they suffer frompoor pull out strength resulting in poor stability, primarily due to thelimited options in machining the contact surfaces. Allograft boneimplants also have variable materials properties and, perhaps mostimportant of all, are in very limited supply. A small but finite risk ofdisease transmission with allograft bone is a factor as well. Inresponse to these problems some developers are attempting to use poroustantalum-based metal constructs, but these have met with limited successowing to the poor elastic modulii of porous metals.

A typical titanium alloy spinal fusion device is constructed from ahollow cylindrical and externally threaded metal cage-like constructwith fenestrations that allow communication of the cancellous hosttissue with the hollow core, which is packed with morselized bone graftmaterial. This design, constrained by the materials properties oftitanium alloys, relies on bony ingrowth into the fenestrations inducedby the bone graft material. However, the titanium-based structure canform a thin fibrous layer at the bone/metal interface, which degradesbone attachment to the metal. In addition, the hollow core into whichthe graft material is packed may have sub-optimal stress transmissionand vascularization, thus eventually leading to failure to incorporatethe graft. Mechanical stability, transmission of fluid stress, and thepresence of osteoinductive agents are required to stimulate the ingrowthof vascular buds and proliferate mesenchymal cells from the cancelloushost tissue into the graft material. However, most titanium-based spinalfusion devices in use today have end caps or lateral solid walls toprevent egress of the graft outwardly from the core and ingress ofremnant disc tissue and fibroblasts into the core.

Autologous (patient) bone fusion has been used in the past and has atheoretically ideal mix of osteoconductive and osteoinductiveproperties. However, supply of autologous bone material is limited andsignificant complications are known to occur from bone harvesting.Moreover, the costs associated with harvesting autograft bone materialare high, requiring two separate incisions, with the patient having toundergo more pain and recuperation due to the harvesting andimplantation processes. Additionally, autologous cancellous bonematerial has inadequate mechanical strength to support intervertebralforces by itself, whereby the bone material is normally incorporatedwith a metal-based construct.

Ceramic materials provide potential alternative structures for use inspinal fusion implant devices. In this regard, monolithic ceramicconstructs have been proposed, formed from conventional materials suchas hydroxyapatitie (HAP) and/or tricalcium phosphate (TCP). See, forexample, U.S. Pat. No. 6,037,519. However, while these ceramic materialsmay provide satisfactory osteoconductive and osteoinductive properties,they have not provided the mechanical strength necessary for theimplant.

Thus, a significant need exists for further improvements in and to thedesign of bone grafts such as spinal fusion implant devices,particularly to provide a high strength implant having high boneingrowth and fusion characteristics, together with substantialradiolucency for effective and facilitated post-operative monitoring.

Hence, it is an object of the present invention to provide an improvedbone graft such as an interbody spinal fusion implant or cage made froma bio-compatible open pore structure, which has a radiolucency similarto that of the surrounding bone. It is also an object of the presentinvention to provide a substrate of high bio-mechanical strength forcarrying biological agents which promote inter-vertebral bone ingrowth,healing and fusion. It is a further objective of the present inventionto provide an interbody fusion device which has mechanical propertiesthat substantially match that of natural bone, by using ceramicconstruct materials rather than metal.

SUMMARY OF THE INVENTION

In accordance with the invention, an improved bone graft such as aspinal fusion cage is provided for human implantation into the spacebetween a pair of adjacent vertebrae, following removal of disc materialbetween endplates of the adjacent vertebrae, to maintain the adjacentvertebrae in a predetermined and substantially fixed spaced relationwhile promoting interbody bone ingrowth and fusion. In this regard, theimproved spinal fusion cage of the present invention is designed for usein addressing clinical problems indicated by medical treatment ofdegenerative disc disease, discogenic lower back pain, andspondylolisthesis.

The improved bone graft, as embodied in the form of the improved spinalfusion cage, comprises a porous or open-celled substrate block formedfrom a ceramic composition having a relatively high bio-mechanicalstrength and load bearing capacity. A preferred composition for theceramic substrate block comprises a two phase alumina-zirconiacomposition having a suitable size and shape for seated implantationinto the inter-vertebral space, with generally planar or appropriatelyshaped faces seated respectively upon the endplates of the adjacentvertebrae. In a preferred form, the substrate block comprises analumina-zirconia composition with an alumina to zirconia ratio rangingfrom about 100% (i.e., 2:1) to about 25% (i.e., 1:4). More preferably,the substrate block composition comprises about 10% to about 20% byvolume zirconia in alumina. Either yttria stabilized zirconia (about 2.5to about 5 mol % yttria in zirconia) or ceria stabilized zirconia (about2.5 to about 15 mol % ceria in zirconia) is preferably used for thezirconia phase. The substrate block is porous, having a porosity ofabout 10% to about 50% by volume with open pores distributed throughoutand a pore size range of from about 5 to about 500 microns. In apreferred form, the porosity of the substrate block is gradated from afirst relatively low porosity region emulating or mimicking the porosityof cortical bone to a second relatively higher porosity region emulatingor mimicking the porosity of cancellous bone.

The porous substrate block is internally and externally coated with abio-active surface coating material selected for relatively highosteoconductive and osteoinductive properties, such as a hydroxyapatiteor a calcium phosphate material.

The thus-formed bone graft can be made in a variety of shapes and sizesto suit different specific implantation requirements. Preferred shapesinclude cylindrical or partly cylindrical with substantially flatopposite ends and a tapered or lordotic cross section to suit therequired curvature of the inter-vertebral space, in the case of a spinalfusion device. The exterior surface of the cylindrical body may includethreads or ribs for facilitated and secure screw-in placement, forexample, between adjacent vertebrae. Alternative preferred shapesinclude a generally rectangular block which may also include serrationsor the like on one or more exterior faces thereof, and/or may have atapered or lordotic cross section for improved fit into theinter-vertebral space. The bone graft may desirably include a posteriorend having an engagement means such as a threaded socket for releasibleengagement with a suitable insertion tool. In addition, the bone graftmay also include one or more laterally open recesses or bores forreceiving and supporting osteoconductive bone graft material, such asallograft (donor) or autograft (patient) material.

Further alternative bone graft configurations may include controlledporosity gradations to define a relatively low porosity first region orregions substantially emulating cortical bone, to define a high strengthloading bearing zone or strut for absorbing impaction and insertionload, in combination with one or more relatively high porosity secondregions substantially emulating cancellous bone for contacting adjacentpatient bone for enhanced bone ingrowth and fusion.

The resultant bone graft exhibits relatively high mechanical strengthfor load bearing support, for example, between adjacent vertebrae in thecase of a spinal fusion cage, while additionally and desirably providinghigh osteoconductive and osteoinductive properties to achieve enhancedbone ingrowth and interbody fusion. Importantly, these desirablecharacteristics are achieved in a structure which is substantiallyradiolucent so that the implant does not interfere with post-operativeradiographic monitoring of the fusion process.

In accordance with a further aspect of the invention, the bone graft mayadditionally carry one or more therapeutic agents for achieving furtherenhanced bone fusion and ingrowth. Such therapeutic agents may includenatural or synthetic therapeutic agents such as bone morphogenicproteins (BMPs), growth factors, bone marrow aspirate, stem cells,progenitor cells, antibiotics, or other osteoconductive, osteoinductive,osteogenic, or any other fusion enhancing material or beneficialtherapeutic agent.

Other features and advantages of the invention will become more apparentfrom the following detailed description, taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a perspective view illustrating an externally threadedradiolucent bone graft such as a spinal fusion cage having an externallythreaded and generally cylindrical shape in accordance with onepreferred for of the invention;

FIG. 2 is a perspective view showing the spinal fusion cage of FIG. 1 inexploded relation with a tip end of an associated insertion tool for usein implanting the spinal fusion cage into the inter-vertebral spacebetween a pair of adjacent vertebrae in a patient;

FIG. 3 is a perspective view showing implantation of the spinal fusioncage into the inter-vertebral space;

FIG. 4 is a perspective view depicting one alternative preferred andgenerally cylindrical bone graft such as a spinal fusion cage;

FIG. 5 is a perspective view depicting a further alternative preferredform of the invention, comprising a generally rectangular bone graftsuch as a spinal fusion cage having at least one serrated externalsurface;

FIG. 6 is a perspective view showing still another alternative preferredform of the invention, comprising a generally rectangular bone graftsuch as a spinal fusion cage;

FIG. 7 is a perspective view showing a further alternative preferredform of the invention, comprising a generally rectangular bone graftsuch as a spinal fusion cage with localized regions of varying porosity;

FIG. 8 is a transverse sectional view taken generally on the line 8—8 ofFIG. 7;

FIG. 9 is another perspective view showing a modified preferred form ofthe invention, comprising a generally rectangular bone graft such as aspinal fusion cage with alternative localized regions of varyingporosity; and

FIG. 10 is a transverse sectional view taken generally on the line 10—10of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the exemplary drawings, a radiolucent bone graft such as aspinal fusion cage referred to generally in FIGS. 1-3 by the referencenumeral 10 is provided for seated implantation between a pair ofadjacent patient bones such as spinal vertebrae 12 (FIG. 3) to maintainthe vertebrae in spaced relation while promoting interbody bone ingrowthand fusion. In general, the improved bone graft 10 comprises abio-compatible ceramic substrate having a porous construction to definean open lattice conducive to interbody bone ingrowth and fusion, whileproviding a strong mechanical load bearing structure analogous to theload bearing properties of cortical and cancellous bone. Thisopen-celled ceramic substrate is coated internally and externally with abio-active surface coating selected for relatively strongosteoconductive and osteoinductive properties, whereby the coatedceramic substrate provides a scaffold conducive to cell attachment andproliferation to promote interbody bone ingrowth and fusion attachment.The ceramic substrate may also carry one or more selected therapeuticagents suitable for bone repair, augmentation and other orthopedic uses.

FIGS. 1-3 illustrate the improved bone graft in the form of an improvedspinal fusion cage 10 in accordance with one preferred embodiment, inthe shape of a generally cylindrical body having external ribs formedthereon in the shape of a screw thread 14. The opposite ends of thiscylindrical body are generally flat, with a posterior end 16 shown toinclude a shallow diametrically extending notch 18 in combination withan axially centered and rearwardly open threaded socket 20 forreleasible engagement with an insertion tool 22 (FIGS. 2-3), as will bedescribed in more detail. The cylindrical body is shown further toinclude a laterally open recess or bore 24 for receiving and supportingmorselized bone graft material 26 (FIG. 2), such as allograft bonematerial from a third party donor, or autograft bone material from thepatient.

The preferred ceramic substrate composition comprises a relatively highstrength ceramic substrate block. In accordance with one preferred formof the invention, this substrate block comprises a two phasealumina-zirconia composition having a controlled porosity and having asuitable size and shape for seated implantation, such as into theinter-vertebral space in the case of the spinal fusion cage 10. In apreferred form, the composition comprises an alumina-zirconiacomposition with an alumina to zirconia ratio ranging from about 100%(i.e., 2:1) to about 25% (i.e., 1:4). More preferably, the substratecomposition comprises about 10% to about 20% by volume zirconia inalumina. Either yttria stabilized zirconia (about 2.5 to about 5 mol %yttria in zirconia) or ceria stabilized zirconia (about 2.5 to about 15mol % ceria in zirconia) are preferably used for the zirconia phase.

The alumina-zirconia composition is processed to provide a homogeneousdistribution of the two phases, and sintering temperatures arecontrolled to provide a particle size of about 0.5 micron or less in thesintered state. The resultant ceramic substrate block has a porosityranging from about 2% to about 80% by volume, and preferably from about10% to about 50% by volume, with pore sizes ranging from about 5 micronsto about to about 1,500 microns, and preferably from about 100 to about500 microns. In the preferred form, the pores are arranged for fluidcontinuity therebetween, and with a rough porous outer surface of largeor extended surface area. Moreover, in the preferred form, the pores arearranged with a variable porosity gradient to define a first region ofrelatively low or reduced porosity (less than about 5%) mimickingcortical bone structure and a second region of relatively large orincreased porosity (ranging from about 30% to about 80%) mimickingcancellous bone structure. In one preferred configuration, the outer orexternal surfaces of the reticulated substrate block comprise the firstor low porosity region for improved load bearing capacity, while theinterior surfaces of the substrate block comprises the second or highporosity region mimicking cancellous bone for enhance bone ingrowth andfusion. This material used to form the substrate block exhibits asubstantially optimal combination of flexural strength (greater thanabout 500 MPa [mega-Pascals]) and fracture toughness (greater than about5 Mpam^(0.5) [mega-Pascal root meter]). These strengths are as measuredfor standard bend bars and single edge notched beam (SENB) specimens,per ASTM E-1304 and C-1162 procedures.

This high strength ceramic substrate block is surface-coated internallyand externally with a bio-active organic or inorganic surface coatingmaterial selected for relatively strong osteoconductive andosteoinductive properties to provide a nutrient rich environment forcellular activity to promote interbody bone ingrowth and fusionattachment. Preferred surface coating materials comprise a resorbablematerial such as hydroxyapatite or a calcium phosphate ceramic.Alternative glassy (amorphous) materials having a relatively richcalcium and phosphate composition may also be used, particularly whereinsuch materials incorporate calcium and phosphate in a ratio similar tonatural bone or hydroxyapatite. Such glassy compositions may comprise apartially or fully amorphous osteoinductive material comprising acomposite of a glass and osteoinductive calcium compound, with acomposition varying from about 100% glass to 100% osteoinductive calciumcompound. The surface coating may also comprise autologous bone marrowaspirates.

The resultant bone graft 10 thus comprises the substrate block formedfrom the high strength ceramic material having bio-mimetic propertiesand which is nonresorbable, or slowly or infinitely slowly resorbablewhen implanted into the patient, in combination with the bio-activesurface coating which is comparatively rapidly resorbable to promoterapid and vigorous bone ingrowth activity.

The substrate block may also advantageously be coated or impregnatedwith one or more selected therapeutic agents, for example, such asautologous, synthetic or stem cell derived growth factors or proteinsand growth factors such as bone morphogenic protein (BMP) or a precursorthereto, which further promotes healing, fusion and growth. Alternativetherapeutic agents may also include an antibiotic, or naturaltherapeutic agents such as bone marrow aspirates, and growth factors orprogenitor cells such as mesenchymal stem cells, hematopoietic cells, orembryonic stem cells, either alone or as a combination of differentbeneficial agents.

The resultant illustrative spinal fusion cage 10 exhibits relativelyhigh bio-mechanical strength similar to the load bearing characteristicsof natural bone. In addition, the spinal fusion cage 10 exhibitsrelatively strong osteoconductive and osteoinductive characteristicsattributable primarily to the surface coating, again similar to naturalbone. Importantly, the fusion cage 10 is also substantially radiolucent,so that the fusion cage does not interfere with post-operativeradiological analysis of interbody bone ingrowth and fusion.

FIG. 2 illustrates the spinal fusion cage 10 with the bone graftmaterial 26 carried within the laterally open bore 24, to achievefurther enhanced bone ingrowth and fusion when the device is implantedinto the inter-vertebral space. The insertion tool 22 includes athreaded tip 28 at a forward or nose end thereof for threaded engagementinto the threaded socket 20 at the posterior end of the fusion cage 10.A locking abutment 30 is also provided at the forward end of theinsertion tool 22 for seated engagement into the shallow notch 18 on thefusion cage. Thus assembled, the insertion tool 22 is used by a surgeonas viewed in FIG. 3 to seat the spinal fusion cage 10 into the spacebetween a selected adjacent pair of vertebrae 12, with the fusion cagebearing against a substantial portion of the end plates of the twovertebrae on opposite sides of the inter-vertebral space. This insertionprocess may be accompanied by appropriate rotation of the insertion tool22 so that the abutment 30 thereon rotatably drives the fusion cage 10to the desired and securely seated implanted position. Then, theabutment 30 can be retracted from the fusion cage 10 followed byback-rotation of the threaded tip 28 for release from the fusion cage.

FIGS. 4-6 illustrate alternative configurations for improved bone graftssuch as spinal fusion cages constructed in accordance with the presentinvention, it being recognized and understood that the bone graft can beconstructed in a wide range of different geometric sizes and shapes.FIG. 4 shows a spinal fusion cage 110 having a generally cylindricalshape similar to the fusion cage 10 shown and described in FIGS. 1-3,but wherein the external screw thread 14 is omitted. As shown, thespinal fusion cage 110 (FIG. 4) has an open-celled structure defined bya high strength porous ceramic substrate block (as previously described)coated with the bio-active surface coating material, but wherein thecylindrical exterior surface is defined by the relatively roughopen-lattice substrate structure having sufficient surface discontinuityand high surface area for optimized contact with cancellous bone toachieve substantially optimized bone ingrowth. FIG. 5 shows anotheralternative spinal fusion cage 210 comprising a surface-coated highstrength porous ceramic substrate block (formed as previously described)with a generally rectangular block configuration to include at least oneexterior surface to include a plurality of serrations 214 for securelymechanical locking with adjacent patient bones such as adjacentvertebrae 12 at opposite sides of the inter-vertebral space. FIG. 6depicts an alternative rectangular block-shaped spinal fusion cage 310comprising another surface-coated high strength porous ceramic substrateblock (formed as previously described) having a narrower cross sectionaldimension and an exterior surface defined by the relativelyrough-textured open-lattice substrate (similar to FIG. 4). Each of theembodiments depicted in FIGS. 1-3 and FIGS. 4-6 has a height dimensionand may be tapered or lordotic in shape (as shown in FIG. 6) forenhanced anatomical fit, for example, into the inter-vertebral space orthe like.

FIGS. 7-10 depict further alternative preferred forms of the invention,wherein the porosity gradient within the high strength porous ceramicsubstrate block is controllably varied to provide desirable load bearingbenefits consistent with bone ingrowth and fusion. More particularly,FIGS. 7-8 show a generally rectangular bone graft such as a spinalfusion cage 410 having a tapered height dimension in theanterior-posterior direction. The substrate block is formed with thefirst region 40 of relatively low porosity substantially mimickingcortical bone to extend across the anterior and posterior faces andfurther to include at least one interconnecting load bearing strut 42shown in the illustrative drawings to extend centrally in ananterior-posterior direction within the body of the substrate block. Theremainder of the substrate block comprises the second portion 44 ofrelatively high porosity substantially mimicking cancellous bone. Theharder first region 40 including the central strut 42 beneficiallyprovides a hard and strong load bearing structure capable ofwithstanding impaction and insertion forces in the anterior-posteriordirection without damage to the implant, while the softer second region44 presents an exposed and large surface area for substantiallyoptimized interknitting ingrowth and fusion with adjacent patient bone.In a spinal fusion cage application, the medial-lateral faces of theimplant are advantageously defined by the softer second region 44,wherein these regions are thus exposed to traditional medial-lateralX-ray imaging for post-operative radiological analysis of theimplant/bone interface. Persons skilled in the art will recognize andappreciate that alternative configurations for the load bearing strut orstruts 42 may be used, such as an X-shaped strut configuration extendingin a cranial-caudial direction, in combination with or in lieu of theexterior faces 40 and/or the anterior-posterior central strut as shown.

FIGS. 9-10 show another generally rectangular bone graft such as aspinal fusion cage 510 having the tapered height dimension in theanterior-posterior direction. The substrate block is formed with thefirst region 40 of relatively low porosity substantially mimickingcortical bone to extend circumferentially about the substrate block insurrounding relation to the second portion 44 of relatively highporosity substantially mimicking cancellous bone. The harder firstregion 40 once again beneficially provides a hard and strong loadbearing structure capable of withstanding impaction and insertion forcesin the anterior-posterior direction without damage to the implant, andadditionally provided high strength vertical load carrying capacity. Bycontrast, the softer second region 44 is vertically exposed to present alarge surface area for substantially optimized interknitting ingrowthand fusion with adjacent patient bone. The external corners of thesubstrate block may incorporate laterally open slots 46 (FIG. 9) forconvenient engagement by and manipulation with a suitable surgical tool(not shown).

In both of the embodiments of FIGS. 7-10, the substrate block comprisesa high strength porous ceramic as previously described, and is coatedwith the bio-active surface coating material, again as previouslydescribed, to enhance bone ingrowth and fusion. The substrate block mayalso include one or more therapeutic agents. Persons skilled in the artwill recognize and appreciate that the relatively low and high porosityregions 40 and 44 shown in FIGS. 7-10 will be integrally joined by asuitable albeit relatively narrow gradient region wherein the porositytransitions therebetween.

The improved bone graft such as the illustrative spinal fusion cage ofthe present invention thus comprises an open-celled substrate blockstructure which is coated with a bio-active surface coating, and has thestrength required for the weight bearing capacity required of a fusiondevice. The capability of being infused with the appropriate biologiccoating agent imparts desirable osteoconductive and osteoinductiveproperties to the device for enhanced interbody bone ingrowth andfusion, without detracting from essential load bearing characteristics.The radiolucent characteristics of the improved device beneficiallyaccommodate post-operative radiological examination to monitor the boneingrowth and fusion progress, substantially without undesirableradio-shadowing attributable to the fusion cage. The external serrationsor threads formed on the fusion cage may have a variable depth to enablethe base of the device to contact the cortical bone for optimal weightbearing capacity. In addition to these benefits, the present inventionis easy to manufacture in a cost competitive manner. The invention thusprovides a substantial improvement in addressing clinical problemsindicated for medical treatment of degenerative disc disease, discogeniclow back pain and spondylolisthesis.

The bone graft of the present invention provides at least the followingbenefits over the prior art:

[a] a porous osteoconductive scaffold for enhanced fusion rates;

[b] a bio-mimetic load bearing superstructure providing appropriatestress transmission without fatigue failure;

[c] a pore structure and size suitable for ingrowth and vascularization,

[d] the ability to absorb and retain an osteoinductive agent such asautologous bone marrow aspirate or BMPs;

[e] bio-inert and bio-compatible with adjacent tissue and selected forease of resorption;

[f] fabricatable and machinable into various shapes;

[g] sterilizable; and

[h] low manufacturing cost.

A variety of further modifications and improvements in and to the spinalfusion cage of the present invention will be apparent to those personsskilled in the art. In this regard, it will be recognized and understoodthat the bone graft implant can be formed in the size and shape of asmall pellet for suitable packing of multiple implants into a boneregeneration/ingrowth site. Accordingly, no limitation on the inventionis intended by way of the foregoing description and accompanyingdrawings, except as set forth in the appended claims.

What is claimed is:
 1. A spinal fusion cage for implantation between andfusion with adjacent vertebrae, comprising: a porous ceramic substrateblock having a relatively high strength corresponding substantially withnatural cortical and cancellous bone, said porous ceramic substrateblock having a first region of relatively low porosity substantiallymimicking natural cortical bone, and a second region of relatively highporosity substantially mimicking cancellous patient bone; and abio-active and resorbable surface coating applied to said substrateblock, said surface coating having osteoconductive and osteoinductiveproperties to promote interbody bone ingrowth and fusion attachment withthe adjacent vertebrae.
 2. The spinal fusion cage of claim 1 whereinsaid porous ceramic substrate block comprises a ceramic structure formedfrom alumina, zirconia, or composition thereof.
 3. The spinal fusioncage of claim 2 wherein said porous ceramic substrate block comprises analumina-zirconia composition having an alumina to zirconia ratio rangingfrom about 100% to about 25%.
 4. The spinal fusion cage of claim 2wherein said porous ceramic substrate block comprises analumina-zirconia composition having from about 10% to about 25% zirconiain alumina.
 5. The spinal fusion cage of claim 1 wherein said porousceramic substrate block is formed from a material with a flexuralstrength greater than about 500 mega-Pascals, and a fracture toughnessgreater than about 5 mega-Pascal root meter.
 6. The spinal fusion cageof claim 1 wherein said porous ceramic substrate block has a porosityranging from about 2% to about 80% by volume, and further wherein thepore size ranges from about 5 microns to about 1,500 microns.
 7. Thespinal fusion cage of claim 6 wherein said porosity ranges from about10% to about 50% by volume, and wherein the pore sizes range from about100 microns to about 500 microns.
 8. The spinal fusion cage of claim 1wherein said first region has a porosity of less than about 5%, andwherein said second region has a porosity ranging from about 30% toabout 80%.
 9. The spinal fusion cage of claim 1 wherein said firstregion is generally disposed on the exterior of said substrate block,and said second region is generally disposed on the interior surfaces ofsaid substrate block.
 10. The spinal fusion cage of claim 1 wherein saidfirst region is generally disposed on anterior and posterior surfaces ofsaid substrate block and further defines at least one structural loadbearing strut extending through said substrate block between saidanterior and posterior surfaces, said second region including anextended exposed surface area for contacting the adjacent vertebrae. 11.The spinal fusion cage of claim 10 wherein said second region issubstantially exposed on medial and lateral surfaces of said substrateblock.
 12. The spinal fusion cage of claim 1 wherein said first regioncircumferentially surrounds and supports said second region, said secondregion including an extended exposed surface area for contacting theadjacent vertebrae.
 13. The spinal fusion cage of claim 1 wherein saidfirst region comprises at least one structural load bearing strutextending through said substrate block, wherein said second regionincludes an extended exposed surface area for contacting the adjacentvertebrae.
 14. The spinal fusion cage of claim 1 wherein said substrateblock further includes means for facilitated grasping and manipulationwith a surgical instrument for implantation.
 15. The spinal fusion cageof claim 1 wherein said bio-active surface coating is selected from thegroup consisting of hydroxyapatite and calcium compounds.
 16. The spinalfusion cage of claim 1 wherein said bio-active surface coating comprisesan organic coating material.
 17. The spinal fusion cage of claim 16wherein said organic coating material is selected from the groupconsisting of autologous bone marrow aspirates, bone morphogenicproteins, growth factors and progenitor cells, and mixtures thereof. 18.The spinal fusion cage of claim 17 wherein said progenitor cells includemesenchymal stem cells, hematopoietic cells, and embryonic stem cells.19. The spinal fusion cage of claim 1 wherein said porous ceramicsubstrate block is substantially radiolucent.
 20. The spinal fusion cageof claim 1 further including a therapeutic agent carried by saidsubstrate block.
 21. The spinal fusion cage of claim 20 wherein saidtherapeutic agent comprises a natural or synthetic osteoconductive orosteoinductive agent.
 22. The spinal fusion cage of claim 1 wherein saidsubstrate block has a rough exterior surface.
 23. The spinal fusion cageof claim 1 wherein said substrate block has a ribbed exterior surface.24. The spinal fusion cage of claim 1 wherein said substrate block has alaterally open bore formed therein, and further including anosteoconductive material supported within said bore.
 25. The spinalfusion cage of claim 24 wherein said osteoconductive material comprisesmorselized bone graft material.
 26. A spinal fusion cage forimplantation between and fusion with adjacent vertebrae, comprising: aporous ceramic substrate block having a relatively high strengthcorresponding substantially with natural cortical and cancellous bone,said porous ceramic substrate block having a variable porosity gradientsubstantially mimicking natural cortical and cancellous bone; and abio-active and resorbable surface coating applied to said substrateblock, said surface coating having osteoconductive and osteoinductiveproperties to promote interbody bone ingrowth and fusion attachment withadjacent patient bone.
 27. A spinal fusion cage for implantation betweenand fusion with adjacent vertebrae, comprising: a porous ceramicsubstrate block having a relatively high strength correspondingsubstantially with natural cortical and cancellous bone; and abio-active and resorbable surface coating applied to said substrateblock, said surface coating having osteoconductive and osteoinductiveproperties to promote interbody bone ingrowth and fusion attachment withthe adjacent vertebrae; said bio-active surface coating being internallyand externally applied to said porous ceramic substrate block.
 28. Aspinal fusion cage for implantation between and fusion with adjacentvertebrae, comprising: a porous ceramic substrate block having arelatively high strength corresponding substantially with naturalcortical and cancellous bone; and a bio-active and resorbable surfacecoating applied to said substrate block, said surface coating havingosteoconductive and osteoinductive properties to promote interbody boneingrowth and fusion attachment with the adjacent vertebrae; saidbio-active surface coating comprising a partially or fully amorphousosteoinductive material including a glass and osteoinductive calciumcompound.
 29. A spinal fusion cage for implantation between and fusionwith adjacent vertebrae, comprising: a porous ceramic substrate blockhaving a relatively high strength corresponding substantially withnatural cortical and cancellous bone; and a bio-active and resorbablesurface coating applied to said substrate block, said surface coatinghaving osteoconductive and osteoinductive properties to promoteinterbody bone ingrowth and fusion attachment with the adjacentvertebrae; the pores formed within said substrate block being insubstantially open fluid communication sufficient to transmit fluidpressure therebetween.
 30. A spinal fusion cage for implantation betweenand fusion with adjacent vertebrae, comprising: a substrate block havinga relatively high strength corresponding substantially with naturalcortical and cancellous bone, said substrate block having a first regionof relatively low porosity substantially mimicking natural corticalbone, and a second region of relatively high porosity substantiallymimicking cancellous patient bone; and a bio-active and relativelyrapidly resorbable surface coating applied to said substrate block, saidsurface coating having osteoconductive and osteoinductive properties topromote interbody bone ingrowth and fusion attachment with the adjacentvertebrae; said substrate block being relatively nonresorbable orresorbable at a rate substantially less than said surface coating.
 31. Aspinal fusion cage for implantation between and fusion with adjacentvertebrae, comprising: a substrate block including at least one loadbearing strut having high strength structural characteristics, saidsubstrate block having a first region of relatively low porositycomprising said strut and substantially mimicking natural cortical bone,and a second region of relatively high porosity substantially mimickingcancellous patient bone; and a bio-active and resorbable surface coatingcarried by said at least one strut, said surface coating havingosteoconductive and osteoinductive properties to promote interbody boneingrowth and fusion attachment with adjacent patient bone.
 32. The bonegraft of claim 31 wherein said at least one strut is formed from aporous ceramic material.